1. Bioreactors

2. On the basis of mode of operation, a bioreactor may be classified as batch, fed batch or continuous (e.g. Continuous stirred-tank reactor model). An example of a continuous bioreactor is the chemostat.
Organisms growing in bioreactors may be suspended or immobilized. The simplest, where cells are immobilized, is a Petri dish with agar gel. Large scale immobilized cell bioreactors are:
►moving media
►packed bed
►fibrous bed
►membrane

3. Bioreactor design
Bioreactor design is a complex engineering task. Under optimum conditions, the microorganisms or cells are able to perform their desired function with 100 percent rate of success.
The bioreactor's environmental conditions like gas (i.e., air, oxygen, nitrogen, carbon dioxide) flow rates, temperature, pH and dissolved oxygen levels, and agitation speed/circulation rate need to be closely monitored and controlled .
Most industrial bioreactor manufacturers use vessels, sensors and a control system networked together.

4. Bioreactor for cellulosic ethanol research

5. Bioreactor
A bioreactor may refer to any device or system that supports a biologically active environment.
In one case, a bioreactor is a vessel in which is carried out a chemical process which involves organisms or biochemically active substances derived from such organisms. This process can either be aerobic or anaerobic.

6. Bioreactors
Biological factors include the characteristics of the cells, their maximum specific growth rate, Monod constant, yield coefficient, pH range and temperature range.
The productivity of a fermentation is determined by the mode of operation of the fermentation process; eg. the advantages of fed-batch and continuous fermentations over batch fermentations.

7. Bioreactors- Introduction
Likewise mass transfer, in particular, oxygen transfer was highlighted as an important factor which determined how a reactor must be designed and operated.
Cost was also described as an important consideration. The larger the reactor or the faster the stirrer speed, the greater the costs involved.
In this lecture, we shall look into how bioreactors are designed to meet cost, biological and engineering needs

8. 2. Methods of Aeration
A bioreactor is a reactor system used for the culture of microorganisms. They vary in size and complexity from a 10 ml volume in a test tube to computer controlled fermenters with liquid volumes greater than 100 m3. They similarly vary in cost from a few cents to a few million dollars.
In the following sections we will compare the following reactors
Standing cultures
Shake flasks
Stirred tank reactors
Bubble column and airlift reactors
Fluidized bed reactors

9. 3. Standing cultures
In standing cultures, little or no power is used for aeration. Aeration is dependent on the transfer of oxygen through the still surface of the culture.

10. Standing cultures
The rate of oxygen transfer will be poor due to the small surface area for transfer. Standing cultures are commonly used in small scale laboratory systems in which oxygen supply is not critical. For example, biochemical tests used for the identification of bacteria are often performed in test- tubes containing between 5-10 ml of media.
T-flasks used in the small scale culture of animal cells are another example of a standing culture. T-flasks are normally incubated horizontally to increase the surface area for oxygen transfer.

11. The surface aeration rate in standing cultures can be increased by using large volume flasks.
The following photograph shows a 250 ml Erlenmeyer flask containing ml of medium and a 3 litre "Fernback" flask containing 1 litre of medium.
Note how the latter has a large surface area.

12. Standing cultures
Large Pyrex flasks are used for the small scale production of fermented products. One example is Kombucha tea which is a tea brewed by mixture of yeasts and acetic acid bacteria.
Standing culture aeration is not restricted to the laboratory.
In some countries, where the availability of electricity is unreliable, citric acid is produced using surface culture techniques.
In these cultures, the Aspergillus niger mycelia are grown on the surface of liquid media in large shallow trays.
The medium is neither gassed nor agitated.

13. Aspergillus niger mycelia

14. Standing cultures
Aerobic solid substrate fermentations are another example of standing cultures. In these fermentations, the biomass is grown on solid biodegradable substrates such as water softened bran, rice or barley.
The solids may be continuously or periodically turned over to improve aeration and to regulate the culture temperature. One example of a commercial scale, solid substrate fermentation is the production of koji by Aspergillus oryzae on soya beans which is part of the soya sauce process.
Another is mushroom cultivation. Considerable research is currently being invested into the feasibility of producing biochemicals by solid substrate fermentations.

15. 4. Shake flasks

16. Shake flasks
Shake flasks are commonly used for small scale cell cultivation.
Through continuous shaking of the culture fluid, higher oxygen transfer rates can be achieved as compared to standing cultures.
Shaking continually breaks the liquid surface and thus provides a greater surface area for oxygen transfer.
Increased rates of oxygen transfer are also achieved by entrainment of oxygen bubbles at the surface of the liquid.

17. Shake flasks
Although higher oxygen transfer rates can be achieved with shake flasks than with standing cultures, oxygen transfer limitations will still be unavoidable particularly when trying to achieve high cell densities.
The rate of oxygen transfer in shake flasks is dependent on the
shaking speed
the liquid volume
shake flask design

18. Shake flasks O2 Transfer
kLa decreases with liquid volume
kLa is higher when baffles are present
kLa
kLa
kLa
kLa
kLa increases with liquid surface area

19. Shake flasks O2 Transfer
The kLa will increase with the shaking speed.
At high shaking speeds, bubbles become entrained into the medium to further increases the oxygen transfer rate.
The presence of baffles in the flasks will further increase the oxygen transfer efficiency, particularly for orbital shakers.
The following photographs show how baffles increase the level of gas entrainment in a shake flask being shaken in an orbital shaker at 150 rpm

20. Unbaffled flask
Baffled flask

21. Shake flasks O2 Transfer
Note the high level of foam formation in the baffled flask due to the higher level of gas entrainment.
The same improvement in oxygen transfer is not as evident with horizontal reciprocating shakers.
The appropriate liquid volume is determined by the flask volume. For example, for a standard 250ml flask, the liquid volume should not exceed 70 ml while for a 1 litre flask, the liquid volume should be less than 200 ml.
Larger liquid volumes can be used with wide based flasks

22. 5. Mechanically stirred bioreactors

23. Mechanically stirred bioreactors
For aeration of liquid volumes greater than 200 ml, various options are available.
Non-sparged mechanically agitated bioreactors can supply sufficient aeration for microbial fermentations with liquid volumes up to 3 litres.
However, stirring speeds of up to 600 rpm may be required before the culture is not oxygen limited.
In non-sparged reactors, oxygen is transferred from the head-space above the fermenter liquid. Agitation continually breaks the liquid surface and increases the surface area for oxygen transfer.

24. (5.1) Mechanically stirred reactors - Sparged stirred tank bioreactors
For liquid volumes greater than 3 litres, air sparging is required for effective oxygen transfer.
The introduction of bubbles into the culture fluid by sparging, leads to a dramatic increase in the oxygen transfer area.
Agitation is used to break up bubbles and thus further increase kLa.
Sparged fermenters required significantly lower agitation speeds for aeration efficiencies comparable to those achieved in non-sparged fermenters.
Air-sparged fermenters can have liquid volumes greater than 500,000 litres.

26. 6. Bubble driven bioreactors
Sparging without mechanical agitation can also be used for aeration and agitation. Two classes of bubble driven bioreactors are bubble column fermenters and airlift fermenters.
Bubble driven bioreactors are commonly used in the culture of shear sensitive organisms such as moulds and plant cells. An airlift fermenter differs from bubble column bioreactors by the presence of a draft tube which provides better mass and heat transfer efficiencies.
Airlift fermenters are however considerably more expensive to construct than bubble column reactors. There are several designs for air-lift fermenters although the most commonly used design is one with a central draft tube.

27. Bubble driven bioreactors

28. Bubble driven bioreactors
An airlift fermenter differs from bubble column bioreactors by the presence of a draft tube which provides
better mass and heat transfer efficiencies
more uniform shear conditions.
Bubble driven fermenters are generally tall with liquid height to base ratios of between 8:1 and 20:1.
The tall design of these fermenters leads to high gas hold- ups, long bubble residence times and a region of high hydrostatic pressure near the sparger at the base of the fermenter.
These factors lead to high values of kLa and Co* thus enhanced oxygen transfer rates

29. 7. Airlift bioreactors
An airlift fermenter differs from bubble column bioreactors by the presence of a draft tube.
The main functions of the draft tube are to:
Increase mixing through the reactor The presence of the draft tube enhances axial mixing throughout the whole reactor
Reduce bubble coalescence. This presumably occurs due to circulatory effect that the draft tube induces in the reactor. The circulation occurs in one direction and hence the bubbles also travel in one direction.

30. Airlift bioreactors
Small bubbles lead to an increased surface area for oxygen transfer.

31. Airlift bioreactors
Equalise shear forces throughout the reactor. Major reason why the productivity of cells grown in airlift bioreactors have higher productivities than those grown in stirred tank reactors.

32. Airlift bioreactors The major disadvantages of air-lift fermenters are
- high energy requirements
- excessive foaming
- cell damage due to bubble bursting; particularly with animal cell culture

33. (7.1) Airlift bioreactor Air-riser and down-comer
An air-lift reactor is divided into three regions:
- the air-riser
- down-comer
- disengagement zone.

34. Airlift bioreactor

35. Airlift bioreactor
The region into which bubbles are sparged is called the air- riser. The air-riser may be on the inside or the outside of the draft-tube. The latter design is preferred for large scale fermenters as it provides better heat transfer efficiencies.
The rising bubbles in the air-riser cause the liquid to flow in a vertical direction. To counteract these upward forces, liquid will flow in a downward direction in the down-comer. This leads to liquid circulation and thus improved mixing efficiencies as compared to bubble columns.
The enhanced liquid circulation also causes bubbles to move in a uniform direction at a relatively uniform velocity. This bubble flow pattern reduces bubble coalescence and thus results in higher kLa values as compared to bubble column reactors.

36. (7.2) Airlift bioreactors - Disengagement zone

37. Airlift bioreactors - Disengagement zone
The roles of the disengagement zone are to
add volume to the reactor,
reduce foaming and
minimise recirculation of bubbles through the down comer.

38. Airlift bioreactors - Disengagement zone
The sudden widening at the top of the reactor slows the bubble velocity and thus disengages the bubbles from the liquid flow.
Carbon-dioxide rich bubbles are thus prevented from entering the downcomer.
The reduced bubble velocity in the disengagement zone also leads to a reduction in the loss of medium due aerosol formation.
The increase in area will also helps to stretch bubbles in foams, causing the bubbles to burst. The axial flow circulation caused by the draft tube also helps to reduce foaming

39. 8. Packed bed and trickle flow bioreactors
The topic of packed bed bioreactors was discussed in another lecture on immobilisation.

40. Packed bed bioreactors
The rate of mass transfer between the cells and the medium depends on the flow rate and on the thickness of the biomass film on or near the surface of the solid particles.
Packed bed reactors often suffer from problems caused by poor mass transfer rates and clogging. Despite this they are used commercially with enzymatically catalysts and with slowly or non-growing cells.
They are also used in the anaerobic treatment of high strength wastewaters (eg. food processing wastes). Large plastic blocks are used as solid supports for the cells. These blocks have a large surface area for cell immobilization and when packed in the reactor are difficult to clog.

42. Trickle flow bioreactors
Trickle bed reactors are a class of packed bed reactors in which the medium flows (or trickles) over the solid particles. In these reactors, the particles are not immersed in the liquid.
The liquid medium trickles over the surface of the solids on which the cells are immobilized
They are used widely in aerobic treatment of sewage.

43. Trickle flow bioreactors
Oxygen transfer is enhanced by ensuring that the cells are covered by only a very thin layer of liquid, thus reducing the distance over which the dissolved oxygen must diffuse to reach the cells.

44. Trickle flow bioreactors
Because stirring is not used, considerable capital costs are saved.
However, oxygen transfer rates per unit volume are low compared with sparged stirred tank systems.
Trickle flow systems are used widely for the aerobic treatment of sewage.
They are used to polish effluent from the activated sludge or anaerobic digestion process and for the nitrification of ammonia.

45. 9. Fluidised bed reactors

46. Fluidised bed reactors
Fluidised bed bioreactors are one method of maintaining high biomass concentrations and at the same time good mass transfer rates in continuous cultures.
Fluidised bed bioreactors are an example of reactors in which mixing is assisted by the action of a pump. In a fluidised bed reactor, cells or enzymes are immobilised in and/or on the surface of light particles.
A pump located at the base of the tank causes the immobilised catalysts to move with the fluid. The pump pushes the fluid and the particles in a vertical direction. The upward force of the pump is balanced by the downward movement of the particles due to gravity. This results in good circulation.

47. Fluidised bed reactors
For aerobic microbial systems, sparging is used to improve oxygen transfer rates.
A draft tube may be used to improve circulation and oxygen transfer. Both aerobic and anaerobic fluidised bed bioreactors have been developed for use in waste treatment.
Fluidised beds can also be used with microcarrier beads used in attached animal cell culture.
Fluidised-bed microcarrier cultures can be operated both in batch and continuous mode. In the former the fermentation fluid is recycled in a pump-around loop.

48. Fluidised bed reactors

49. Summary Looked at methods of aeration in different bioreactors
Aeration in standing cultures
Oxygen transfer in shake flasks
Advantages and applications of mechanically stirred bioreactors
Bubble driven bioreactors
Airlift bioreactors
Packed bed and trickle flow bioreactors
Fluidised bed bioreactors